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Oracle Tips by Burleson |
Migration to Oracle
Once the installation of the new Oracle software is complete,
what do we, as DBAs and application developers, do? We migrate. Of
course, we should take the proper steps to ensure that we migrate
like graceful geese flying across a sunset rather than lemmings
dashing forward over a cliff (see Figure 1.20). Naturally, as DBAs,
we prefer the graceful flight across the sunset.
Planning Migration
How do we accomplish this graceful migration? Planning. Planning is
the only way to minimize points of possible failure in your
migration path from Oracle 6.x, Oracle 7.x, or Oracle8/8i to
Oracle. In this section, I will attempt to provide a logical
framework for planning your migration and hopefully shine a light on
the pitfalls that you should avoid along the way
Have you known someone who immediately takes new items (VCRs,
computers, TVs) right out of the box, plugs them in, and never reads
instructions? Later, they fret because the blankity-blank item won’t
do what they want (or, more likely, they don’t know how to make it
do what they want). They didn’t prepare to migrate to their new
appliance. This preparation should have involved reviewing the
installation and operation manuals, performing controlled tests of
new features, and then finally bringing the new appliance into full
use (see Figure 1.21).
Preparing to Migrate
Invariably, other “experts” (using the term loosely) and I will be
asked hundreds of questions about installation and migration to
Oracle that could have been answered if the people asking the
questions had read the documentation and tested new features before
implementation. The point is, you have to take the time to study the
new features sections of the manuals, read the readme.doc, and
review utility scripts for “hidden” changes.
Back when the Oracle8 migration was the rage, I mentioned the
compatible parameter in one of my columns. A couple of weeks later,
Oracle announced a spate of bugs that related to the parameter. Were
the two related? Probably. It demonstrated that people who thought
they had “migrated” to a new version of Oracle hadn’t. Why? Because
of the setting of the parameter, they hadn’t even tested the new
features that depended on a new redo log format! Reading the
documentation before they migrated would have prevented this.
How about the person who buys one of those great new sport utility
vehicles only to discover it won’t fit in their garage? Obviously,
they didn’t check available resources before “migrating” to a new
vehicle. Oracle will take up to 150 percent more space for initial
load than Oracle8 and up to three times the space of Oracle 7.3. Do
you have enough free space? You may want to run both an Oracle and
an Oracle8i (or earlier release) system in parallel. Do you have
enough memory (9i may require twice to three times as much memory as
previous releases (512 megabytes on Linux, for example) or enough
CPU? You will require at least 50 meg of free space in an 8i SYSTEM
tablespace to upgrade to 9i, or the upgrade will fail. You may also
want to take note that the default installation of an Oracle
database contains a SYSTEM tablespace that is near 350 megabytes in
size with only 40 Kilobytes free, you should plan your sizing
accordingly.
Getting back to our geese versus lemmings migration analogy: Which
chose the better migration path (overlooking hunters in this case…)?
Don’t begin your migration without a clear plan of how to get from
your Oracle8i or earlier instance to your Oracle instance. Decide
on your path, preferably one that doesn’t lead you over a cliff!
Review the possible methods to migrate your system; choose the best
for your situation, and then plan, plan, plan! In Oracle
migration, you have the choice between export and import, the Data
Migration GUI tool, and the migration command-line utility. Of
course, there is also the CTAS method, but that is only applicable
to real S-and-M fanatics. In a 6.x version database, you must first
migrate to Oracle7.3.x and then migrate the 7.3.x database into 9i.
In a 7.3.x to 9i, all of the methods are applicable; however, in an
8.1.x to 9i, only the Data Migration Assistant, export and import,
unload, and SQLLOADER or CTAS are available. Figures 1.22 through
1.25 show these migration paths.
Finally, how do you know your migration was successful? Do you have
a test plan? Did you evaluate your existing system prior to
migration to check its performance characteristics? Do you have a
standard set of test cases to run against the old database and the
new? How will you know that the system is functioning correctly
after the migration is “complete?”
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